The invention relates to a method for the preparation of polyolefin foams which are resistant to weathering, especially to ultraviolet radiation in daylight, by peroxide crosslinking and foaming with a chemical blowing agent.
It is known to add organic peroxides and blowing agents to polyolefins. The mixing of the polyolefins with the crosslinking peroxides and blowing agents, plus any common additives, can be performed in a roller mixer or extruder, and they can then be fabricated below the degradation temperature of the peroxide into products such as plates or continuous webs. The crosslinking of the polyolefin is then performed by decomposing the crosslinking agent below the decomposition temperature of the blowing agent. Then, or simultaneously, the temperature is raised to the decomposition range of the blowing agent, so that the polyolefin will be foamed by the gases thus formed.
In addition to the peroxide and the blowing agent, plus any additional adjuvants, foaming adjuvants in the form of sterically hindered phenols or organic phosphorus compounds can be added, in accordance with German Auslegeschrift No. 28 46 646, to the polyolefins. Also, foaming adjuvants such as magnesium compounds, for example, can be used in accordance with German Federal Pat. No. 28 52 187.
The known crosslinked polyolefin foams, however, have the disadvantage of only slight resistance to daylight, especially ultraviolet light, and only slight resistance to weathering.
In their customary use, in which the effect of light and moisture is unavoidable, great embrittlement occurred which greatly reduces the usefulness of the foams for a number of applications.
The embrittlement is not prevented by the addition of phenolic antioxidants, which are known to be stabilizers against the effects of light and heat.
The problem therefore existed of reducing the embrittlement occurring under the influence of light, heat and moisture. For numerous applications there has existed the difficult problem of greatly reducing the embrittlement occurring in conventional crosslinked polyolefin foams, and of lengthening useful life by several times before embrittlement sets in under the influence of light and weather. This object is achieved in accordance with the claims.